1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for its production, and more particularly to an active matrix type liquid crystal display device using a lateral electric field driving system and a method for the production thereof.
2. Description of the Related Art
Since an active matrix type liquid crystal display device (AMLCD) using thin film transistors (TFTs) of field effect type used as switching devices for its pixels is excellent in image quality, AMLCD has been widely used as a monitor of a portable computer or of a desktop computer of a space-saving type.
Recently, particularly, in order to improve the characteristics of view angle, a display method using a lateral electric field has been proposed.
In this proposed method, for example, as disclosed in Japanese Patent Laid-Open No. Hei 6-160878, a pixel electrode and its counter electrode are formed in parallel to each other on the same substrate. In driving the thus formed liquid display device, a voltage is applied between these electrodes to establish an electric field in parallel to the substrates, so that the direction of an anisotropic axis (i.e., director) of the liquid crystal is varied in a plane parallel to a surface of the substrate, whereby a transmission luminous energy is controlled.
Due to this, it is possible for such liquid crystal display device to obtain a pixel capable of having substantially the constant brightness even when viewed from a view point varying in a wide view angle. In contrast with a twisted nematic (TN) system, there is no fear that the liquid crystal director is so energized as to deviate from a surface of the substrate, which causes the relationship between the transmission luminous energy and the applied voltage to considerably vary, the relationship being obtained when the liquid crystal display device is viewed in a direction parallel to the liquid crystal director.
In general, a conventional liquid crystal display device such as one disclosed in the above Japanese Patent Laid-Open No. Hei 6-160878 has a construction in which: the liquid crystal director is homogeneously oriented in a predetermined direction substantially parallel to the substrate (homogeneous orientation); as shown in FIG. 11, the liquid crystal layer is sandwiched between a pair of polarizing plates so as to have their polarizing axes crossed with each other at right angles; and, the polarizing axis of one of the polarizing plates is coincident with the direction of orientation of the liquid crystal director. Due to such construction, it is possible to steadily lower the black level in brightness by displaying the black when no voltage is applied and by rotating the liquid crystal director upon application of a voltage to display the white.
Further, in order to improve the liquid crystal device in coloring problem occurring when the device displaying the white is viewed from an oblique direction, there is proposed a construction in which each of the pixel electrode and its counter electrode is bent into a V-shaped form in each of the pixels, as shown in FIG. 12 (for example, see "O Plus E, January issue for 1997, page 102). In this construction, when a voltage is applied between these electrodes, two electric fields different in direction are established to cause the liquid crystal molecule to twist in two directions.
In its white display mode, when the liquid crystal display device is viewed from an oblique direction C, a region 1 in which the liquid crystal director is oriented to a direction "A" assumes red color. On the other hand, a region 2 in which the liquid crystal director is oriented to a direction B assumes blue color. Since these colors in such two regions are compensated with each other, it is possible to considerably relieve the coloring problem appearing when the liquid crystal device displaying the white is viewed from the oblique direction.
Further, in Japanese Patent Laid-Open No. Hei9-80424, there is proposed a method in which: in the liquid crystal display device of a lateral electric field system, in order to prevent the coloring problem from occurring when a display screen of the liquid crystal display device in its white display mode is viewed from an oblique direction, a double refraction medium is interposed between the opposite substrates or the like and the polarizing plates. In this proposed method, when a maximum driving voltage is applied, i.e., in the white display made, an anisotropic axis of the double refraction medium coincides in direction with each of the liquid crystal director and the polarizing axes of the polarizing plates.
A problem to be resolved by the present invention is as follows:
In the former of the conventional methods, when the liquid crystal display device is in its intermediate tone display mode or in its white display mode, since the regions 1 and 2 having been twisted in two different directions are compensated with each other, it is possible to considerably prevent the coloring problem from occurring when the liquid crystal display device is viewed from an oblique direction.
However, in the black display mode, when the liquid crystal display device is viewed from the oblique direction, in the strictest sense of the word, the direction of the liquid crystal director is not coincident with the direction of the polarizing axis, which results in a retardation, i.e., delay in phase. Due to this, when the liquid crystal display device displaying the black is viewed in the oblique direction, a reddish or a bluish coloring problem occur, depending on the direction from which the display is viewed.
Further, in the conventional method disclosed in Japanese Patent Laid-Open No. Hei9-80424, in the black display mode, the anisotropic axis of the double refraction medium is different in direction from the polarizing axis of the polarizing plate. Due to this, it is impossible for the conventional method to prevent the coloring problem from occurring in the black display mode.